Numerous multilamp photoflash arrangements with various types of sequencing circuits have been described in the prior art, particularly in the past few years. Series and parallel-connected lamp arrays have been shown which are sequentially fired by mechanical switching means, simple electrical circuits, switching circuits using the randomly varied resistance characteristics of the lamps, arc gap arrangements, complex digital electronic switching circuits, light-sensitive switching means and heat-sensitive switching devices which involve melting, fusing or chemical reaction in response to the radiant energy output of a proximate flashed lamp. The present invention is concerned with an improved radiant-energy-activated switching means useful in a relatively inexpensive photoflash unit of the disposable type. In particular, the present switching means is particularly advantageous in photoflash arrays employing high voltage type lamps adapted to be ignited sequentially by successively applied high voltage firing pulses from a source such as a camera-shutter-actuated piezoelectric element.
A currently marketed photoflash unit of the last-mentioned type is described in U.S. Pat. No. 3,894,226 and referred to as a flip flash. The unit comprises a planar array of eight high voltage type flashlamps mounted on a printed circuit board with an array of respectively associated reflectors disposed therebetween. The lamps are arranged in two groups of four disposed on the upper and lower halves, respectively, of the rectangular shaped circuit board. A set of terminal contacts at the lower end of the unit is provided for activation of the upper group of lamps, while a set of terminal contacts at the top of the unit is operatively associated with the lower group of four lamps. The application of successive high voltage pulses (e.g., 500 to 4000 volts from, say, a piezoelectric source controlled by the shutter of a camera in which the array is inserted) to the terminal contacts at the lower end of the unit causes the four lamps at the upper half of the array to be sequentially-ignited. The array may then be turned end for end and again inserted into the camera in order to flash the remaining four lamps.
The flip flash circuit board comprises an insulating sheet of plastic having a pattern of conductive circuit tracers, including the terminal contacts, on one side. The flashlamp leads are electrically connected to these circuit traces by means of eyelets secured to the circuit board and crimped to the lead wires. The circuitry on the board includes six printed, normally open, connect switches that chemically change from a high to low resistance, so as to become electrically conducting, after exposure to the radiant heat energy from an ignited flashlamp operatively associated therewith. The purpose of these switches is to promote lamp sequencing and one-at-a-time flashing. The four lamps of each group are arranged in parallel, with three of the four lamps being connected in series with a respective thermal connect switch. Initially, only the first of the group of four lamps is connected directly to the voltage pulse source. When this first lamp flashes, it causes its associated thermal connect switch (which is series connected with the next, or second lamp) to become permanently conductive to high voltage. Because of this action, the second lamp of the group of four is connected to the pulse source. This sequence of events is repeated until all four lamps have been flashed.
One type of radiation actuated connect switch is described in U.S. Pat. No. 3,459,488 of Schroder et al, in which a paste globule containing a metal compound dissociates to form an electrical conductive bridge in response to actinic light radiation. In U.S. Pat. No. 3,458,270 of Ganser et al, the use of silver oxide in a polyvinyl binder is taught as a normally open radiant energy switch. Upon radiant heating, the silver oxide decomposes to give a metallic silver residue which is electrically conductive. More recently silver carbonate has been favored over silver oxide for this use because of its lower conductivity toward high voltage prior to thermal actuation. Some other related patents include the following: U.S. Pat. Nos. 3,598,511, 3,726,631, 3,728,067, 3,728,068, 3,692,995, 3,774,020, 3,532,931, 3,459,487, 3,668,421, 3,562,508, 3,443,875, 3,951,582, 3,969,065 and 3,969,066.
Also, it is known that the above-mentioned normally open (N/O) radiation-responsive switches utilizing a paste containing silver oxide or silver carbonate tend to react vigorously to conversion from a normally open condition to a conductive condition. As a result, the switching device is susceptible to be "blown off" the circuit board and thereby fail to provide the desired low resistance electrical path.
One attempt to eliminate the above-described "blow off" problem is suggested in U.S. Pat. No. 4,080,155 issued to Sterling. Therein, a mixture of silver oxide and a suitable carbon-containing silver salt are utilized with non-conductive particulate solids, such as glass beads, to provide a slurry or paste suitable for use in fabricating a N/O switch for a sequentially operable multilamp photoflash array.
Although the above-described N/O switches employing silver oxide and a silver-salt with non-conductive particulate solids have been used with varying results, it has been found that such switches tend to require a relatively large quantity of silver. For example, a silver salt content of more than 50% by weight is not uncommon. Moreover, the cost of switches utilizing such large amounts of silver tends to be prohibitive or at least very undesirable.